Marketed products commonly require some type of marking on the product for commercial, regulatory, cosmetic or functional purposes. A mark is defined as contiguous region or area on the surface of the article which contrasts visually with the adjacent surface. Desirable attributes for marking include consistent appearance, durability, and ease of application. Appearance refers to the ability to reliably and repeatably render a mark with a selected shape and uniform color and optical density. Durability is the quality of remaining unchanged in spite of abrasion to the marked surface. Ease of application refers to the cost in materials, time and resources of producing a mark including programmability. Programmability refers to the ability to program the marking device with a new pattern to be marked by changing software as opposed to changing hardware such as screens or masks.
Of particular interest is creating marks on coated or painted articles. Articles made of metal or various types of plastics are often painted or otherwise covered in various industrial coatings to protect and change the appearance of the article's surface. Laser ablating the coating in particular patterns to remove the coating and reveal the surface of the article underneath is a desirable way to create a mark on article. Covering an article with two or more layers of coatings and laser ablating a first coating to reveal a second coating underneath is another desirable way to create marks. Marking a product by removing a coating with a laser to reveal the article underneath is discussed in US patent application no. 2008/0152859, inventor Masanori Nagai published Jun. 26, 2008. This method depends upon the brightness of the coating being brighter than the surface of the article. Japanese patent application no. 03-150842, inventor Iwasaki Noboru, published Oct. 29, 1992, describes removing one or more coating layers with a laser to reveal a coating layer underneath.
One thing that these references have in common is that in order to remove a coating without removing materials beneath the coating layer being removed, the laser ablation threshold for the material being removed must be lower than the laser ablation threshold of the material underneath. Laser ablation threshold is the minimum energy required to cause removal of material. This removal may be ablative, where enough energy is put into the material by the laser to cause the material to disassociate into plasma, or thermal, where the material is essentially melted and vaporized, or a combination of the two. Related to the ablation threshold is the damage threshold. The damage threshold is the minimum laser energy required to cause an undesirable change in the appearance of the material. The damage threshold for materials is generally lower and sometimes much lower than the ablation threshold. We define damage as any undesirable change in the appearance of the materials that comprise the article or under lying coating following laser removal of the topmost layer.
In order to cleanly and completely remove a top layer of material leaving the surface of the article undamaged, laser fluence must be greater than the ablation threshold for the overlying material and less than the ablation or damage threshold for the underlying material or surface of the article. In many cases this difference may be small, requiring precise control of laser fluence. In addition, the actual values for ablation and damage thresholds for these materials may vary depending upon location and slight variations in application. Also, actual values for ablation and damage thresholds for these materials may vary as a function of time; as the article is processed, heat generated by the laser removal process may be retained by the article or coatings and influence the ablation or damage thresholds. Therefore, while a single laser fluence may be selected that permits commercially desirable marking to be performed on a particular article at a particular time with a particular laser, any small change in the article or coatings or laser could result in the process no longer producing desirable results. The difference between the minimum fluence that will result in ablation of desired materials and avoid damage to underlying materials is called a process window. In manufacturing it is highly desirable to increase the process window for a particular article and mark to the greatest practical extent in order to handle variations in materials and processing systems without requiring system adjustments which negatively affect system throughput.
What is desired but undisclosed by the art is a reliable and repeatable method for removing material that does not leave undesired material on the surface of the article or require frequent adjustment of the equipment. What is needed then is a method for reliably and repeatably creating marks having a desired appearance on coated articles using a laser to remove a layer of coating without leaving undesired material on the under laying materials while maintaining acceptable system throughput.